The territorial standoff between China and Japan over disputed islands in the East China Sea further escalated today after a Chinese plane was spotted in what Tokyo considers its airspace.

Though the Chinese plane was not a military aircraft, its presence is the latest provocation in a dispute that has affected economic relations between the two countries and comes just three days before Japanese elections.

The Chinese state maritime agency said that the marine surveillance plane was sent to patrol the disputed islands – known as Diaoyu in China and Senkaku in Japan – along with four boats, according to China’s Global Times. Japanese boats also patrolling the disputed area were asked to leave immediately, in line with the Chinese government’s stance, the Global Times reports.

“I want to stress that these activities are completely normal. The Diaoyu and its affiliated islands are China’s inherent territory since ancient times,” Chinese Foreign Ministry spokesman Hong Lei said of the plane. “China requires the Japanese side stop illegal activities in the waters and airspace of the Diaoyu islands.”

International law forbids entering another nation's airspace without permission and gives countries the right to expel unauthorized aircraft with force immediately. In contrast, foreign ships are able sail through a nation's territorial waters as long as it is considered "innocent passage."

The incident also puts further pressure on [Prime Minister Yoshihiko Noda], whose ruling Democratic Party of Japan is likely to face a decisive defeat in Sunday's elections, according to various national polls. Shinzo Abe, who's likely to take his job away, has criticized Mr. Noda for his handling of the territorial issues, and called for a confrontational approach focused on the use of "physical power," rather than diplomacy.

The Japan Daily Press notes that the coast guard has earned fame from the action surrounding the charged island dispute, “causing a surge in job applications and even inspiring a local box-office hit film.” However, the civilian-staffed guard is “being stressed and tested to its limits.”

[I]n order not to escalate the situation, the government isn’t using the military Self Defense Forces. Instead, it has tasked the Coast Guard, composed of civilians and run by the Ministry of Land, Infrastructure, Transport, and Tourism. Although somewhat on par with their Chinese counterparts, the Coast Guard is being stretched thin. Its crew, used to shifts of at most two weeks, are on duty without time off for months. They are also forced to skip crucial training, in a time when they are expected to be able to ward off and restrain offenders both on sea and on remote islands. Ships also receive only temporary repairs instead of much-needed overhauls.

… The situation has become a rallying and unifying point of many of the political parties and candidates running for election in a few days, with promises of allocating funds and beefing up the Coast Guard to empower it to respond to situations and emergencies that the pacifist nation hasn’t faced in decades.

Today was the first time both countries have used planes in the dispute. Reuters reports that Japanese analysts are concerned over the use of aircraft.

"This is serious ... intrusion into Japan's airspace is a very important step to erode Japan's effective control over the area," said Kazuya Sakamoto, a professor at Osaka University.

"If China sends a military plane as a next step, that would really make Japan's control precarious."

Toshiyuki Shikata, a Teikyo University professor and a retired general, said the use of aircraft by both sides was significant.

"Something accidental is more likely to happen with planes than with ships," he said.

Japan bases its claim to the islands, which it calls the Senkaku, on a cabinet decision in January 1895 whereby because there was no trace of anyone else controlling them they were deemed "terra nullius," nobody else's, and Tokyo incorporated them into its territory.

China disputes that claim, pointing to 15th-century accounts of sea voyages by Chinese envoys and a 17th-century map of China's sea defenses, among other documents, to show that "the Diaoyu islands were first discovered, named, and exploited by the Chinese," in the words of a Foreign Ministry statement.